1. Technical Field of the Invention
This invention relates generally to microstrip directional couplers.
2. Description of the Prior Art
Known microstrip directional couplers typically use structures that obtain flatter coupling versus frequency characteristics at the expense of using greater dimensions to accomplish such characteristics. An example, representative of such known directional couplers is the full quarter-wave quasitransverse electromagnetic wave directional coupler. Such a directional coupler, implemented at very high frequency (VHF) on a glass/epoxy printed circuit board requires a coupled region length equal to one foot, which is impractically long for use in power amplifiers. A smaller directional coupler implementation would require forward and reflected port compensation to realize the desired flat coupling response versus frequency characteristic. Moreover, that type of compensation generally results in coupled port mismatching that degrades the directivity (approximately -12 decibels, dB, at VHF) of an uncompensated coupling structure.
Another problem that is common in known VHF microstrip directional couplers is poor directivity. Directivity is a measure of undesirable coupling from a forward port to a reverse port when the forward port is energized. In an ideal four-port backward wave directional coupler, all of the power applied into a primary forward port would appear at the secondary forward port and no power would appear at the secondary reverse port. In such an ideal directional coupler, the directivity would be infinite.
In known microstrip directional couplers, the directivity is poor because the energy above the microstrip structure (the dielectric above the structure being air) propagates faster along the coupler than does the energy propagating in the printed circuit (pc) board substrate. Typical directivity performance for microstrip couplers is 7-13 dB, depending on the coupling and on the frequency of operation. If such a directional coupler is used in a power control loop to sense the forward and reflected power into a load, its directivity will limit the minimum voltage standing wave ratio (VSWR) of the load which can be resolved, as well as power leveling performance into a VSWR.
Prior attempts by those skilled in the art to improve the directivity of directional couplers include the addition of a dielectric overlay, modification of the coupled line shape or the addition of a single, lumped capacitor which bridges each end of the coupled structure. Each of those techniques improves the directivity of tightly-coupled (i.e., less than -10 dB), quarter-wave coupled lines to about -20 dB. However, those techniques become ineffective or impractical to implement when the value coupling is less than -20 dB or when the coupled structure is significantly less than a quarter wavelength long.